Angie Peltier, U of M Extension crops educator and Dean Malvick, U of M Extension corn and soybean plant pathology specialist
One can think of each crop year as a mulligan, a way to start with a clean slate. However, if your 2025 corn crop was seeded into a field that had been in corn recently, and the shorter the rotation away from corn, the greater the likelihood that pathogens able to survive the harsh Minnesota winter are there to challenge this year’s crop. However, not all pathogens that cause disease in Minnesota corn survive in corn residue. Today, we summarize key information about two corn diseases that have been observed in Minnesota in 2025 and request your help in understanding its prevalence.
In non-rotated fields or fields in which there is still residue from a previously infected corn crop remaining on the soil surface, rain splash may spread E. turcicum spores onto leaves in the lower canopy. Or wind may spread spores onto leaves in the upper canopy either from neighboring fields or previously infected plants in the same field.
Symptoms of NLB include long (up to 5 inches in length) canoe-shaped lesions that are parallel to leaf veins and taper at each end (Figure 1). Severe epidemics can result in lesions coalescing until very little leaf area remains healthy and this may lead to early maturity (Figure 2).
Multiple foliar fungicides are labeled for and have good activity against NLB. Foliar fungicides should be considered when one plants a susceptible hybrid, multiple lesions are present at or before tasseling and the weather forecast suggests favorable conditions for repeated cycles of infection. Foliar fungicides are best timed between the tasseling and milk growth stages. Unless you had an abnormally late planted crop of an early relative maturity hybrid for your region of the state, it is likely that your corn has developed beyond these stages of development. Thus it is likely too late for a foliar fungicide to be warranted, particularly under the current input and corn price regime. This is because the probability of a foliar fungicide protecting enough yield potential to at least pay for chemical and application costs decreases as fungicide/application prices increase and corn prices decrease.
One can think of each crop year as a mulligan, a way to start with a clean slate. However, if your 2025 corn crop was seeded into a field that had been in corn recently, and the shorter the rotation away from corn, the greater the likelihood that pathogens able to survive the harsh Minnesota winter are there to challenge this year’s crop. However, not all pathogens that cause disease in Minnesota corn survive in corn residue. Today, we summarize key information about two corn diseases that have been observed in Minnesota in 2025 and request your help in understanding its prevalence.
Northern leaf blight (NLB)
General information
NLB, caused by the endemic, fungal pathogen Exserohilum turcicum, is a disease favored by long periods of leaf wetness and moderate temperatures. This disease is polycyclic, a fancy term for multiple cycles of infection occurring in a single growing season. Therefore, the earlier that initial symptoms occur on plants, the greater the leaf area that can become infected and no longer photosynthesizing and therefore the greater the risk to overall yield potential.In non-rotated fields or fields in which there is still residue from a previously infected corn crop remaining on the soil surface, rain splash may spread E. turcicum spores onto leaves in the lower canopy. Or wind may spread spores onto leaves in the upper canopy either from neighboring fields or previously infected plants in the same field.
Symptoms of NLB include long (up to 5 inches in length) canoe-shaped lesions that are parallel to leaf veins and taper at each end (Figure 1). Severe epidemics can result in lesions coalescing until very little leaf area remains healthy and this may lead to early maturity (Figure 2).
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| Figure 1. Northern leaf blight. (Photo: Dean Malvick) |
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| Figure 2. Severe NLB with coalescing lesions. (Photo: Dean Malvick |
Management
Most seed companies provide ratings for NLB so that one can select hybrids with a high degree of genetic resistance, which is all the more important for the next time that corn is planted in previously infected fields. Genetic resistance is a built-in management practice that does not add to the input bill and should be considered the foundational management practice for NLB.Multiple foliar fungicides are labeled for and have good activity against NLB. Foliar fungicides should be considered when one plants a susceptible hybrid, multiple lesions are present at or before tasseling and the weather forecast suggests favorable conditions for repeated cycles of infection. Foliar fungicides are best timed between the tasseling and milk growth stages. Unless you had an abnormally late planted crop of an early relative maturity hybrid for your region of the state, it is likely that your corn has developed beyond these stages of development. Thus it is likely too late for a foliar fungicide to be warranted, particularly under the current input and corn price regime. This is because the probability of a foliar fungicide protecting enough yield potential to at least pay for chemical and application costs decreases as fungicide/application prices increase and corn prices decrease.
Visit the Crop Protection Network for more information about northern leaf blight.
Symptoms of a southern corn rust infection also exhibit signs of the pathogen, as spores of the pathogen burst through the upper epidermal tissue to form a pustule (Figure 3). Similar to NLB, southern rust is a polycyclic disease. Spores responsible for repeated cycles of infection -called urediniospores- have a light orange appearance and pustules with urediniospores can be closely packed together. As the epidemic progresses, pustules have a darker appearance as darker colored teliospores are produced.
The severity and importance of any southern rust epidemic in Minnesota is a function of the timing of the arrival of inoculum from the south in relation to the growth stage of the crop. Temperatures near 80 degrees and the wet 2025 growing season have combined to result in conditions favoring infection. The earlier that a southern rust epidemic begins, the greater the likelihood of it becoming a yield-limiting epidemic. In many years, spores blow northward late in the summer, causing pustules to form as the corn crop is nearing physiological maturity. Such late infections are unlikely to result in lost yield potential, whereas lost yield potential is much more likely should spores arrive when the corn crop is in the early vegetative growth stages, simply due to the compromised leaf tissue resulting in reduced photosynthetic and yield potential.
The primary means, therefore, of managing a southern rust epidemic is foliar fungicides and there are multiple, effective fungicides labeled for southern rust management. However, similar to NLB, unless your crop was planted abnormally late to a hybrid of early relative maturity, your crop is likely in the dough developmental stage and it is likely too late for a foliar fungicide to be warranted.
Southern rust
Southern rust in 2024 was widespread at high levels of incidence and severity never before seen in Minnesota. So far in 2025 southern rust is being detected in fields scattered across southern Minnesota. For example, University of Extension’s Digital Crop Doc distance diagnostics program received a corn submission from Jackson County, Minnesota early last week in need of visual diagnosis. The picture was of very typical southern rust pustules.General information
Southern rust, caused by the fungus Puccinia polysora, is a periodic disease of Minnesota dent corn as the pathogen is unable to survive over winter in Minnesota. The pathogen therefore must make its way north along the “Puccinia pathway” from the southern US and Mexico where it survives the winter on living corn plants. As the US corn crop is seeded beginning in the southern tier of states progressing northward as temperatures rise in the spring, spores are carried on the wind and in storms that move south to north throughout the center of the continent, causing epidemics along the way. One can follow the geographic jumps in the epidemic nationwide throughout any given growing season on the southern corn rust ipmPIPE map.Symptoms of a southern corn rust infection also exhibit signs of the pathogen, as spores of the pathogen burst through the upper epidermal tissue to form a pustule (Figure 3). Similar to NLB, southern rust is a polycyclic disease. Spores responsible for repeated cycles of infection -called urediniospores- have a light orange appearance and pustules with urediniospores can be closely packed together. As the epidemic progresses, pustules have a darker appearance as darker colored teliospores are produced.
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| Figure 3. Corn leaf exhibiting southern rust pustules in northern Polk County, MN in 2024 (photo: Angie Peltier). |
The severity and importance of any southern rust epidemic in Minnesota is a function of the timing of the arrival of inoculum from the south in relation to the growth stage of the crop. Temperatures near 80 degrees and the wet 2025 growing season have combined to result in conditions favoring infection. The earlier that a southern rust epidemic begins, the greater the likelihood of it becoming a yield-limiting epidemic. In many years, spores blow northward late in the summer, causing pustules to form as the corn crop is nearing physiological maturity. Such late infections are unlikely to result in lost yield potential, whereas lost yield potential is much more likely should spores arrive when the corn crop is in the early vegetative growth stages, simply due to the compromised leaf tissue resulting in reduced photosynthetic and yield potential.
Management
As southern rust is more often an economically important disease in corn grown throughout the southern US, southern rust breeding efforts have largely focused on longer-maturity hybrids not adapted to the Upper Midwest. This is likely why you have been unable to find a southern rust column in seed company catalogues for locally adapted hybrids.The primary means, therefore, of managing a southern rust epidemic is foliar fungicides and there are multiple, effective fungicides labeled for southern rust management. However, similar to NLB, unless your crop was planted abnormally late to a hybrid of early relative maturity, your crop is likely in the dough developmental stage and it is likely too late for a foliar fungicide to be warranted.
Visit the Crop Protection Network for more information on southern rust.
Help us understand disease prevalence!
Please submit pictures of suspected infections through the Digital Crop Doc platform as we are interested in understanding whether 2024 and 2025 are outliers for Minnesota southern rust epidemics or whether this disease is increasing in prevalence over time and is in need of additional study.
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